FileMaker themes are simply a collection of the popular CSS standards plus a few extra attributes which are FileMaker specific. Each element on your FileMaker layouts is defined by these CSS settings. They can be seen when extracting them from the clipboard, and if you know the right location to retrieve and store them, can be defined by any enthusiastic developer.
When FileMaker, now Claris, first introduced themes they likely had the intention of allowing users and/or third parties to create their own. In the time since that initial release it may not be as commonly known, but the location still exists and you can certainly create (or modify) your own theme.
The trick is knowing what's required, what can and can't be done and how the CSS translates to the FileMaker UI widgets. Even still, if you can't take the time to learn every single detail, you can still work with a theme which currently exists and simply tweak that theme to fit your specific needs.
In this video I walk through all the specifics related to customizing one of the existing themes. As long and you get the files into the right location with the right values, then you can modify any theme to your heart's content.
Locations to find existing theme files are here.
Macintosh: [MY_DRIVE_NAME]/Applications/FileMaker Pro.app/Contents/Resources/Themes
Windows: C:\Program Files\FileMaker\FileMaker Pro 19\Themes
Locations to store theme files are here.
Macintosh: [MY_DRIVE_NAME]/Users/[USERNAME]/Library/Application Support/FileMaker/Extensions/Themes
Click the title or link to this article to view the video.